A fuel cell has been proposed as a clean, efficient and environmentally responsible power source. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to an electric vehicle.
One type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte. The PEM fuel cell typically includes three basic components: a cathode electrode, an anode electrode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form the membrane-electrode-assembly (MEA).
The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen, for an electrochemical fuel cell reaction. In the fuel cell reaction, hydrogen gas is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit formed therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where oxygen reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
A pair of electrically conductive end plates or bipolar plates generally sandwich the MEA to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the gaseous reactants (i.e., the H2 & O2/air) of the fuel cell over the surfaces of the electrodes.
As is well understood in the art, the electrolyte membrane within the fuel cell needs to have a certain relative humidity to effectively conduct protons. During operation of the fuel cell, moisture from the fuel cell electrochemical reaction and from external humidification may enter the flow channels of the bipolar plates. Typically, the moisture is forced along the flow channels by a pressure of a gaseous reactant, with this pressure being a primary mechanism for water removal from the flow channels. However, if the pressure is not sufficient, water can accumulate in a phenomenon known as stagnation. Stagnant water can block flow channels and reduce the overall efficiency of the fuel cell. A high degree of water accumulation or stagnation can also lead to fuel cell failure, particularly following a shut-down period under freezing ambient conditions where the accumulated water turns to ice. Both accumulated water and ice may cause gas starvation. Gas starvation is known to result in carbon corrosion when the starved fuel cell is one of a number of fuel cells in the fuel cell stack having an electrical load applied thereto.
Minimizing water stagnation has been possible, for example, by purging the channels periodically with the reactant gas at a higher flow rate or by having generally higher reactant recirculation rates. However, on the cathode of the MEA, this increases the parasitic power applied to the air compressor and reduces overall system efficiency. The use of hydrogen as a purge gas on the anode of the MEA can lead to reduced economy, poorer system efficiency, and increased system complexity.
A reduction in accumulated water in channels can also be accomplished by lessening inlet humidification. However, it is desirable to provide at least some relative humidity in the anode and cathode to hydrate the fuel cell membranes. Dry inlet gas has a desiccating effect on the membrane and can increase a fuel cell's ionic resistance. This method also negatively affects the long-term durability of the membrane.
In fuel cells having ultrathin electrodes, all of the reactions are concentrated in a small space, which leads to severe flooding on the cathode and dry out on the anode. Particularly at temperatures below 60° C., a saturated pressure of the water is too miniscule to effectively transport the water from the electrodes to the flow channels. As a result, a cool start performance of the fuel cell is detrimentally affected and causes a challenge for using the ultrathin electrodes in automotive applications.
Accordingly, it would be desirable to develop a water management feature that transports accumulating water away from fuel cells having ultrathin electrodes, wherein the feature is passive and improves fuel cell performance, particularly at cool-start operating conditions (i.e. about 0° C. to about 60° C.).